1. Field of the Invention
The present invention relates to a tyre for vehicles wheels and, more particularly, to a plurality of improvements referring to tyres suitable for using on wet roads without any prejudice to the performances generally required of a tyre running on dry roads.
2. Description of the Related Art
A tyre in its more general form comprises: a carcass structure including a central crown portion and two axially opposite side walls terminating in a pair of beads for the attachment to the rim of a wheel; a belt structure coaxially associated with the carcass structure and a tread band extended coaxially around the belt structure.
Generally the tread band comprises a raised pattern formed by a plurality of transversal and longitudinal grooves giving rise to a plurality of blocks distributed according to different configurations, for example along a central zone straddling the equatorial plane and in at least two shoulder zones extending in positions axially opposite the central zone.
Tyres comprising a tread band of the “block” type satisfy the requirement of gripping when compared with tyres the pattern of which comprises continuous circumferential ribs that confer good directional capacity to the tyre, but which prove not to possess sufficient gripping above all when adverse weather conditions, such as for example a road surface that is wet, covered with snow or ice, or even a slightly sloping road, have to be faced.
As the tyre rolls, the blocks of the tread band are subjected to a set of thermo-mechanical stresses, which are all the greater the more severe the conditions of use and which result in changes to their geometry and, over varying time periods, to a deterioration in the tyre's performance.
In particular, the contact with the ground during running produces a flexing of the trailing edge of the blocks of the tyre, said edge, by dipping down radially inwardly and moving in the direction opposite the direction of running, produces a restriction of the groove that separates it from the next block. This phenomenon leaves the leading edge of the latter open to a tangential stress which is repeated cyclically at each revolution of the wheel causing a typical uneven and premature wear known as “saw-tooth” wear phenomenon. The recourse to possible solutions comprising blocks of considerable extent, therefore with high resistance to stresses and resultantly less wear, contrasts with the requirement of good adhesion of the tyre to the road. In fact, very extended blocks under running conditions on wet roads leave a continuous film of water in the footprint area between block and ground that is difficult to break with consequent risks of driving instability.
In order to limit the wear due to mobility of the blocks under the footprint area, in some areas of the tread band two rows of blocks separated by a narrow groove was used so that said groove, by closing under the footprint area, produced a reciprocal support between said two rows with a consequent strengthening and reduction in the mobility of the blocks.
A tyre of the “block” type with performances suitable for use on wet roads is described, for example, in patent U.S. Pat. No. 5,240,053.
The tread band of said tyre comprises five circumferential grooves and a plurality of transversal grooves forming complessively six rows of blocks, two central rows, two intermediate rows axially opposite the central ones and two axially external shoulder rows.
The two central rows of blocks are kept apart by a narrow circumferential groove passing through the equatorial plane of the tyre. The width of said central circumferential groove is between ¼ and ⅓ of the width of the other circumferential grooves, while its depth is substantially the same as that of the other circumferential grooves.
The transversal grooves of all the blocks rows arranged to the side of the equatorial plane are inclined to the latter according to angles oriented in the same direction.
Each block of the central rows comprises a notch extending between a lateral circumferential groove and a point at a distance from the narrow central groove. This notch is parallel to the transversal grooves.
Furthermore, in order to reinforce the central blocks in relation to the stresses acting on the central area of the tread band, elastic bridges are provided between circumferentially adjacent blocks.
Each block of the intermediate rows comprises two notches of the same length, both parallel to the transversal grooves.
One of the notches communicates with the axially innermost circumferential groove and the other with the axially outermost circumferential groove and both the notches terminate at a distance from the longitudinal median line of the block, said median line being substantially parallel to the equatorial plane.
As a whole, said pair of notches confers a “S” shape to each block of the intermediate rows.
In consideration of the state of the art, the Applicant posed the problem of improving the known technique and producing a tread band provided with a pattern suitable for both dry ground and wet ground having features suitable to guarantee resistance to the stresses acting on the tyre under conditions of use on straight and cornering runnings, together with good driving stability and suitable noise-free features.
The solution to this problem appeared extremely complex since a tread band is characterized by the so-called “solids/voids” ratio which depends on the amount of rubber taken off the tread band due to the presence of the grooves and, as widely known, to high values of this ratio correspond good gripping features on dry roads but low roadholding values on wet roads.